A number of different systems and methods have been developed for delivering electrical shocks to a patient's heart for the treatment of detected abnormal heart rhythms (arrhythmias). These methods deliver specific waveform shapes or pulse sequences to the heart in order to treat the detected arrhythmia by depolarizing the heart tissue cells. One early waveform is disclosed in U.S. Pat. No. 3,706,313 to Milani et al., which provides a circuit for delivering a "trapezoidal" wave shape for defibrillating the heart by truncating the output of an exponentially decaying capacitor. Others have suggested the use of sequential pulses delivered through multiple pathways such as is described in U.S. Pat. No. 4,708,145 to Tacker, Jr. In Tacker, Jr., a series of rectangular or truncated exponential pulses are delivered to the heart using at least three electrodes. A first pulse is sent through a first pair of the three electrodes and then a second pulse is sent through a second, different pair of the electrodes. Still others have described the use of multiphasic waveforms, such as U.S. Pat. No. 4,637,397 to Jones et al., which describes a triphasic waveform. A triphasic waveform has three pulses of alternating positive and negative polarity. U.S. Pat. No. 3,924,641 to Weiss and U.S. Pat. No. 4,850,357 to Bach, Jr. describe the use of biphasic waveforms.
Defibrillation pulses of the type described above are typically in the range of from about 500 to 1000 volts delivered for a time of from about 2 to 12 milliseconds. Overall energy delivery to the heart for a defibrillation waveform may typically be from about 10 to 40 joules. A monophasic defibrillation waveform may typically be a truncated exponential decay with an initial voltage of about 700 volts and a duration of about 10 milliseconds. A biphasic defibrillation waveform may typically have an initial positive pulse of about 750 volts for a duration of 6 milliseconds and a negative pulse of about 100 to 400 volts for an equal duration. The leading edge voltage of the second pulse of a biphasic waveform may typically be equal to or one half of the trailing edge voltage of the initial pulse which itself depends on the tilt of the pulse. The overall energy delivered is a function of the voltage, duration, tilt and lead impedance.
Another modification of the standard waveform has been suggested by Imran in U.S. Pat. No. 4,768,512. That patent discloses a cardioverting system (defibrillation and cardioversion) in which a truncated exponential waveform is chopped at high frequencies to provide a voltage wave packet formed of a plurality of high-frequency cardioverting pulses with a preferred frequency in excess of 1 kHz.
Tachyarrhythmias, which are rapid but organized heart rhythms, may be treated with cardioversion pulses. These pulses are similar to defibrillation pulses but generally are delivered at lower voltages and are synchronous with the QRS complex. Pulses are delivered synchronously to help avoid accelerating a heart experiencing a ventricular tachyarrhythmia into ventricular fibrillation. Such a cardioverter is disclosed in U.S. Pat. No. 4,384,585 to Zipes. In Zipes, an implantable synchronous intracardiac cardioverter detects intrinsic depolarization of cardiac tissue and provides a shock to the heart in synchrony with the detected cardiac activity at a time when the bulk of cardiac tissue is already depolarized and in a refractory state. Synchronizing defibrillating pulses is not required since the heart is already in fibrillation when such pulses are delivered. Thus, some prior art defibrillators deliver defibrillation shocks to the heart without any correlation or synchronization to the timing of the sensed QRS complex from an electrocardiogram (ECG). Some other prior art devices synchronize such shocks to the QRS complex.
A primary goal in treating a detected tachyarrhythmia with an implantable cardioverter/defibrillator is to ensure delivery of effective therapy while minimizing energy delivery requirements for the defibrillation waveform. Lower voltage therapy is less painful and disruptive to the patient. Also, lower voltage electrical pulses allow for use of smaller batteries and capacitors even where the overall energy delivery is not reduced. Smaller batteries and capacitors result in a smaller implantable defibrillator.